1. Field of the Invention
This invention relates to a pneumatic tire comprising a tread at least a surface of which is made of a foamed rubber composition, and particularly to an improved technique of forming closed cells.
2. Description of Related Art
There is a technique of using a foamed rubber in a tread rubber of a so-called studless tire so as to obtain good performances on icy roads such as a driving property, a steering property and the like, in running on snow and/or icy roads (see: JP-A-62-283001, JP-A-63-235921, JP-A-63-90403 etc.).
It is desirable that a wear resistance property and the performances on icy roads are more improved in the studless tire applied with the foamed rubber.
However, in a conventional technique of forming closed cells when a closed cell content is decreased in order to improve the wear resistance property, the performances on icy roads is lowered, on the contrary when the closed cell content is increased in order to improve the performances on icy roads, the wear resistance property is degraded.
In the conventional technique of forming the closed cells it is impossible to control closed cell content and closed cell size separately. And, when the closed cell content is decreased, the closed cell size simply becomes small and even though the wear resistance property is improved, the performances on icy roads is degraded. On the contrary when the closed cell content is increased, the closed cell size becomes large and the performances on icy roads is improved but the wear resistance property is degraded.
It is, therefore, an object of the invention to improve the wear resistance property and the performances on icy roads together.
In order to attain the object, the inventors have studied hard and developed a technique capable of controlling the closed cell content and the closed cell size separately and accomplished this invention.
Therefore, in the present invention only the closed cell size can be made large or small while the closed cell content is kept constant.
Concretely, a wear phenomenon is caused by the fact that cracks generated at the closed cells grow and conjugate one another. This can be solved by making a distance between closed cells larger and suppressing conjugation of cracks. That is, while the closed cell content is kept optimum, the closed cell size is made larger to decrease a density of a number of the closed cells and to make the distance between closed cells larger, so that the wear resistance property can be improved. On the other hand, it is necessary to increase the closed cell content in order to improve the performances on icy roads.
Further, in a heavy duty pneumatic tire it is more required that the wear resistance property is good, and in a pneumatic tire for a passenger car it is more required that the performances on icy roads is good.
Meantime, a vulcanization reaction and a foaming reaction simultaneously occur and a difference of their reaction rates influences the closed cell size. That is, when the formation of the cells by foaming proceeds faster than a formation of three-dimensional bond through vulcanization, the cell size becomes large, on the contrary when the vulcanization reaction proceeds faster than the formation of the cells, the cell size becomes small.
Since the foaming assistant used in the present invention accelerates the vulcanization reaction as well, the relative reaction rate between vulcanization reaction and the foaming reaction can be controlled by regulating a compounding amount of the foaming assistant based on the foaming agent amount, and hence the closed cell content and the closed cell size can be controlled separately by regulating the foaming agent amount and a ratio of the foaming assistant amount to the foaming agent amount.
According to an aspect of the invention, there is a provision of a pneumatic tire comprising a tread at least a ground contacting portion of which is made of a foamed rubber composition, which comprises matrix rubber comprising at least one rubber ingredient selected from the group consisting of natural rubber and synthetic rubbers and closed cells which is formed by compounding a foaming agent and a foaming assistant which comprises sodium benzenesulfinate or a combination of sodium benzenesulfinate and urea and being vulcanized.
In preferable embodiments of the invention, the foamed rubber composition further comprises short fibers which are solid and/or hollow and/or a filler, at least one part of the closed cells may be covered with a coat layer made from the short fiber material, the foaming agent contains azodicarbonamide, the compounding amount of the foaming agent is 1-20 parts by weight based on 100 parts by weight of the rubber ingredient and the foaming assistant is of 15-200% by weight based on the amount of the foaming agent compounded, a content of the closed cells is of 1-100%, or a mean size of the closed cells is of 30-200 xcexcm. Further, the pneumatic tire is used for a passenger car and as the filler at least one selected from the group consisting of carbon black and silica is compounded in the foaming rubber composition in an amount of 30-80 parts by weight based on 100 parts by weight of the rubber ingredient, or the pneumatic tire is used as a heavy duty pneumatic tire and as the rubber ingredient comprises not less than 50% by weight of natural rubber and as the filler carbon black is compounded in an amount of 30-80 parts by weight based on 100 parts by weight of the rubber ingredient, in the foaming rubber composition.
Though the rubber tends to decrease in its hardness owing to the cells"" existence, in case of including short fibers in the rubber matrix, the hardness of the rubber can be increased. In this case, the short fibers may be solid and/or hollow. When the short fibers are hollow, a hollow inside the fiber in addition to the cell formed by foaming effectively function on improvement of the performances on icy roads.
Further, when the fiber is made of a material which melts at any temperatures during vulcanization, since the gas formed through foaming reaction tends to gather at a melted fiber portion which is lower in viscosity than the rubber matrix portion, the cell becomes covered with the melted fiber material. In this case, when the fiber is hollow, the bigger closed cell may be formed by combining the gas formed through foaming reaction to the hollow inside the fiber (FIG. 3). On the other hand, when the fiber is a material which does not melt at any temperatures during vulcanization, the fiber maintains the form of the fiber even after the vulcanization (FIG. 4).
Moreover, the foaming agent preferably contains azodicarbonamide, and is compounded in an amount of 1-20 parts by weight based on 100 parts by weight of the rubber ingredient and the foaming assistant of 15-200% by weight based on the amount of foaming agent compounded.
Further, the content of the closed cells is preferably 1-100% even when hollow fibers are compounded. And the mean size of the closed cells in the rubber matrix without being covered with the fiber material is more preferably 30-200 xcexcm. Cells those which have not been covered with the fiber material have a form of substantial sphere, while cells those covered with the fiber material has an elongated shape.